Electronic surface structure of n -ML Ag/Cu(111) and Cs/ n -ML Ag/Cu(111) as investigated by 2PPE and STS

Abstract

We investigated the electronic structure of epitaxially grown silver films on Cu(111) with and without adsorption of cesium by means of scanning tunneling spectroscopy and two-photon photoemission. This system has been chosen as a model system to engineer and measure the dynamics of charge-transfer processes between an adsorbate and a heterogeneous substrate. Special emphasis has been laid on the investigation of the energy shift of the Shockley-type surface state and an excited cesium resonance as a function of Ag film thickness. For the cesium resonance we observe an increase in line width with increasing layer thickness.PACS 68.37.Ef; 68.55.Jk; IntroductionThe dynamics of excited electrons on adsorbatecovered surfaces determines many surface processes, in particular photon-and electron-stimulated reactions. The efficiency of these reaction mechanisms depends in a crucial way on the lifetime of intermediate electronic states. Recently, it has been demonstrated experimentally and theoretically that adsorbed alkali atoms on noble metals are an ideal model system to study the dynamics of charge-transfer processes between surface and chemisorbed adsorbates [1][2][3]. In a theoretical work, Borisov et al. showed that the substrate band structure is the most important parameter for the specific relaxation dynamics of the excited alkali state [3]. This knowledge opens the possibility to modify the adsorbate-substrate coupling and, hence, the dynamics of charge-transfer processes by the use of a properly adjusted heterogeneous substrate. Metallic thin-film overlayers, such as Co/Cu(111) or Ag/Cu(111), are ideal model substrates for this kind of study.Cu(111) has a gap of ∼ 5 eV around the Fermi level in the projected band structure at the Γ -point (L 2 -L 1 gap). As a result of the termination of the crystal at the surface an occupied Shockley-type surface state appears in this gap, which represents a quasi-two-dimensional electron gas. The existence of these states has been proposed by Shockley [4] and was exper- imentally identified first by Gartland and Slagsvold [5]. The localization of this state right at the surface makes it in general an ideal probe for the determination of surface conditions and properties [6]. In the few-monolayer (ML) regime a metallic overlayer of e.g. Na [7], Pd [8] or Ag [9] influences the binding energy of the surface state of Cu(111). For thicker films (more than 10 ML) discrete quantum well states (QWS) appear in the band gap, which have been observed by means of photoemission [10]. The energy of these states is determined by the film thickness and can be tuned quasi-continuously. Therefore, by adjusting the energetic position of relevant electronic states by means of metallic thin-film overlayers one has the potential to engineer the dynamics of charge-transfer processes on adsorbate-covered surfaces and, hence, can modify specific properties of chemical surface reactions such as efficiency and reaction channels.The paper is organized as follows: in the first part we presen...

Comparative study of the interfaces of graphene and hexagonal boron nitride with silver

“…Even if the energy position of the spectral feature in the h-BN islands is similar in both systems, there is a large difference in the onset on the Ag/Cu(111) areas with respect to the bare silver. Previous work has shown that the Ag film thickness on Cu(111) determines the energy shift of the Shockley-type surface state [47]. Accordingly, we conclude that the copper surface is covered by 3 ML of silver (see Fig.…”

“…Table I (111), spatially modulated following the moiré superstructure (−0.84 eV in the valleys and −1.14 eV on the hills) [28]. Once the silver intercalates, the surface state band minimum is shifted smoothly to higher values in the Ag/Cu(111) areas (e.g., at −235 mV for 3 ML of Ag) [47]. On the h-BN regions, the surface state depopulates and the band onset appears at similar energies as for h-BN It is interesting to note that the Ag thickness allows us to tune the properties of the system in a controlled manner.…”

The influence of electron confinement, quantum size effects, and film morphology on the dispersion and the damping of plasmonic modes in Ag and Au thin films

“…STM and LEED measurements show a (9 Â 9) reconstruction for 1 ML Ag [306][307][308][309][310][311][312][313][314]. Also for this system, ARPES experiments [312,[315][316][317][318] have revealed the existence of well-defined Ag 5sp-derived QWS even for rather high film thickness, with subsequent significant changes in the dynamical screening properties with respect to the case of Ag single crystals. Due to the presence of a surface projected energy gap in the Cu(1 1 1) band structure, QWS on Ag/Cu(1 1 1) are characterized by standing wave patterns [314], while on the contrary standing waves are not supported on Ag/ Ni(1 1 1) due to fact that Ag QWS do not arise within the energy gap of Ni(1 1 1).…”

Electronic states of moiré modulated Cu films

“…Since the centroid of this band lays above E F , the surface state shifts upward in energy with respect to its position in unstrained Cu(111) crystals. As the film thickness increases, the structural distortions in the topmost Cu layers attenuate and, correspondingly, the energy shift decreases (see the discussion on the surface properties of the Ag/Cu(111) system in [32]). …”